Any discussion of the background art throughout the specification should in no way be considered as an admission that such background art is prior art, nor that such background art is widely known or forms part of the common general knowledge in the field.
Optical absorption spectroscopy is an active branch of research which finds important applications in biochemical sensing. The fundamental challenge is to attain ever lower detection thresholds. Towards this end, optical resonators are excellent transducers for amplifying the effects of small optical absorption and loss.
By comparing the intensity of an optical beam before and after the beam is passed through a substance, the optical density or absorption of that substance can be determined. Shot noise on the measurement beam sets the ultimate resolution limit to which this measurement can be made.
Methodologies adopting radio-frequency modulation for detecting vibrational overtones of gas-phase molecules have recorded shot-noise limited performance at 1.5×10−13/√Hz [see for example J. Ye, L. S. Ma and J. L. Hall, Ultrasensitive detections in atomic and molecular physics: demonstration in molecular overtone spectroscopy, J. Opt. Soc. Am. B, 15, p. 615 (1998)]. These frequency modulation (FM) spectroscopy techniques are powerful for species with absorption linewidths of ˜1 GHz.
For molecules in liquids, on the other hand, the spectral line is greatly broadened with a homogeneous profile. This can be compounded by overlapping of profiles from different spectral lines, resulting in an absorption continuum which can extend up to tens of THz. With conventional laser modulation technology, this absorption continuum appears as a broadband loss to the spectral components of the frequency modulated laser, which renders frequency modulation spectroscopy techniques (such as those described by J. Ye et. al. referenced above) ineffective.
The most widely investigated cavity enhanced technique suitable for broadband loss is time-domain ring-down spectroscopy. However, this technique typically yields poor sensitivity due to limited optical duty cycle and large effective noise bandwidth.
A highly sensitive alternative for measuring absorption in liquids has recently been proposed [see Timothy McGarvey, Andre Conjusteau and Hideo Mabuchi, Finesse and sensitivity gain in cavity-enhanced absorption spectroscopy of biomolecules in solution, Opt. Ex., 14, 22, 10441-10451 (2006)] where the laser is locked on resonance, while the reflected power is continually monitored for changes in cavity loss, The sensitivity achieved, however, was still greater than 200 times the shot noise limit.
It is an object of the present invention to substantially overcome or at least ameliorate one or more of the above disadvantages, or at least to provide a useful alternative.